Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, a common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogenous catalysts into substantially equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for the acetone co-product.
Thus, a process that avoids or reduces the use of propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam, and nylon 6.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is via benzene hydroalkylation in which benzene is contacted with hydrogen in the presence of a catalyst such that a portion of the benzene is converted into cyclohexene which then reacts with the remaining benzene to produce the desired cyclohexylbenzene. One such method is disclosed in U.S. Pat. No. 6,037,513, in which the catalyst comprises a molecular sieve of the MCM-22 type and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. This reference also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide, which is then decomposed to the desired phenol and cyclohexanone co-product in roughly equimolar amounts.
Several technical challenges not seen in the cumene-based Hock process exist in producing phenol via cyclohexylbenzene. One such challenge is that non-negligible amounts of by-products, including phenylcyclohexanols and phenylcyclohexanones, are generated during the oxidation step. To improve process economics, it would be desirable to develop a process for converting these phenylcyclohexanols and phenylcyclohexanones to useful products. In seeking to develop such a process, the present inventors have found that phenylcyclohexanols and phenylcyclohexanones can be converted back to cyclohexylbenzene in high yield by a combination of dehydration and hydrogenation. However, investigation of this process has also demonstrated that, particularly with the 2-phenyl isomers, such as 2-phenyl-1-cyclohexanol and 2-phenyl-1-cyclohexanone, the dehydration step is accompanied by isomerization of the 2-phenylcyclohexene intermediate to 1-methylcyclopentenylbenzene and cyclopentenylmethylbenzene. On hydrogenation, these products are converted to 1-methylcyclopentylbenzene and cyclopentylmethylbenzene, which are difficult to remove from the hydrogenation effluent by either physical or chemical techniques.